There is no 90 day multiple entry visa. There is a 6 month METV visa granting you multiple entries, each entry is 60 days and can be extended once per entry another 30 days. You need to enter before the expiry/enter before date on the visa for your final entry and stamp.
Anonymous participant just write the short letter specifying which DTV you want and why the work you do qualifies for that DTV. Nobody is asking for a signed, sealed, stamped, letterhead certificate.